1. Field of the Invention
The present invention relates to an image sensor comprising a matrix of photosensitive cells arranged in lines and columns and formed in a CMOS-type technology.
2. Discussion of the Related Art
FIG. 1 schematically shows an image sensor formed of a matrix of pixels 1 receiving through an objective 2 the image of a distant object field of vision 3. The image of a point A located in the middle of the object plane will form substantially in the middle of matrix 1. The image of a point B located at the edge of the object plane will form at the edge of matrix 3.
FIG. 2 is a cross-section view of a substantially central photosensitive cell 6 of an image sensor formed on a substrate 7, for example, silicon. Photosensitive cell 6 is associated with a portion of the surface of substrate 7 which, in top view, generally has the shape of a square or of a rectangle. Photosensitive cell 6 comprises an active photosensitive area 8 generally corresponding to a photodiode capable of storing a quantity of electric charges according to the received light intensity. Substrate 7 is covered with a stacking of transparent insulating layers 9, 11, 12, 13 which may be, as an example, alternatively silicon oxide and silicon nitride. Conductive tracks 14, formed between adjacent insulating layers, and conductive vias 16, formed between two conductive tracks, especially enable addressing photosensitive area 8 and collecting electric signals provided by photosensitive area 8. Conductive tracks 14 and conductive vias 16 are generally metallic. As an example, aluminum, tungsten, copper, and metal alloys may be used. Such materials are opaque and possible reflective. In a color sensor, a color filter element 17, for example, an organic filter, is arranged on the stacking of insulating layers 9, 11, 12, 13 at the level of photosensitive cell 6. The elements of color filter 17 are generally covered with a planarized equalization layer 18 which defines an exposition surface 19 exposed to light.
The maximum light sent by objective 2 (FIG. 1) on the portion of exposition surface 19 associated with photosensitive cell 1 must be directed towards photosensitive active area 8. For this purpose, a microlens 21 is arranged on equalization layer 18, opposite to photosensitive area 8 to focus the light rays on photosensitive area 8. The paths of two rays of light R1, R2 are schematically shown as an example for photosensitive cell 6. Conductive tracks 14 and conductive vias 16 are arranged to avoid hindering the passing of the rays of light. Microlens 21 is for example obtained by covering equalization layer 18 with a resin. The resin is etched to delimit distinct resin blocks, each resin block being arranged substantially opposite to a photosensitive area 8. The resin blocks are then heated. Each resin block then tends to deform by reflow to obtain a convex external surface 22. As an example, for a photosensitive cell 6 with a 4-μm side and for a distance on the order of from 8 to 10 μm between a microlens 21 and the associated photosensitive area 8, the maximum thickness of microlens 21 is approximately 0.5 μm.
Photosensitive area 8 only covers a portion of the surface of substrate 7 associated with photosensitive area 6. Indeed, a portion of the surface is reserved to devices for addressing and reading photosensitive area 8. For clarity, these devices have not been shown in FIG. 2.
FIG. 3 is a cross-section view of a photosensitive cell 6 of an image sensor located at the edge of the pixel matrix. Two rays R1′, R2′ have been shown as an example. It can be observed that the focusing of rays R1′ and R2′ is performed on the side of photosensitive area 8. Thus, a portion at least of the light spot provided by microlens 21 does not hit photosensitive area 8 and the image edges will appear, for a same lighting, darker than the image center. This results in a loss of peripheral sensitivity due to the offset of the image projected on the peripheral portions of the photosensitive cell matrix.
It is constantly attempted to decrease the dimensions of image sensors to be able to integrate an always-increasing number thereof on a same surface of a substrate. This results, for photosensitive cell 6, in a decrease in lateral dimension d of photosensitive cell 8. However, distance T between microlens 21 and photosensitive area 8 generally does not significantly vary. The ratio between distance T and lateral dimension d thus tends to increase. Microlens 21 must thus be less converging to enable focusing on photosensitive area 8. Microlens 21 must thus be thinner, which requires that the resin layer deposited on equalization layer 18 must be thinner. In the steps of reflow of the resin blocks etched in the resin layer, it becomes in practice difficult to obtain a microlens with a shape enabling it to have the light rays converge properly. Further, the increase of this ratio tends to increase the size of the ray of light at the level of the photosensitive area which can then become wider than the photosensitive area. This results in a loss in the available light intensity. A third limitation of the current manufacturing process results from the presence of conductive tracks 14 and of conductive vias 16 which can be obstacles and hinder the passing of the light rays.